FCM client library benefits over GCM (for push delivery service)

We are developing a push notification delivery service (something like 1Signal) in Iran. We are using Firebase as the push provider as google says, but cannot be solved for the client library using FCM or GCM. We will not announce delivery or click of a notification, or flexibility in handling notifications for developers.

I've read official docs and SO questions like this which are not very helpful and the fact that I haven't found a push delivery service using FCM in their SDKs adds more confusion.

So a specific and technical comparison would be great or tell me why should I use FCM SDK in my SDK for push delivery system ?